--- Quote from: jwatte on February 15, 2013, 12:41:59 PM ------ Quote ---if not chain, how do u suggest i drive both wheels with a single motor..?--- End quote ---

You said you'd use two motors. I suggest driving the left rear wheel with one motor, and the right rear wheel with the other motor, and leaving the front wheels un-driven. If you can add servo-based steering, you'll be better at turning, but you can probably do without it.

Or just put in four motors. Motors are cheaper than transmissions and linkages these days... Look at, for example, the Wild Thumper series for design ideas.

--- End quote ---

if i let two front wheels undriven, i'll have issues it making 90 degree turns while standing.. :s can you suggest what exact motors should i use? a picture will help :P

To calculate RPM of the motor use formula: RPM = DesiredVelocity / WheelCircumference.

As for 4WD, why not to go with 2 drive wheels and replace other 2 wheel by a caster wheel, that way You can perform fluid turns.

jwatte:
Right, there are two options:

1) Use four wheels like a model car. Either steer the wheels in front, or drive each wheel individually and do "tank steering." The Wild Thumper does the latter -- check it out here: http://www.pololu.com/catalog/product/1565That, in turn, uses these kinds of motors: http://www.pololu.com/catalog/product/2274

2) Use two big wheels, and two smaller wheels that are just "limp" and ideally pivot. Casters, balls, cart wheels, whatever you want to call them. Turning the big wheels different directions will pivot the robot in place. With only two motors, this is the easiest to implement.

3) You probably want motors with built-in encoders, so you can tell whether one wheel is going faster than the other and compensate, to go straight.

4) You don't need the full weight * radius amount of torque, as long as you don't want to accelerate wildly (and can make soft start/stop part of your control.) Wheels turn very easily, at least if they are narrow and hard.

If you haven't gotten any motors yet, those motors are a fine recommendation for 6V-8V systems. If you are doing a 12V-16V system, try these instead: http://www.pololu.com/catalog/product/1444

You will need some motor controllers to drive them, too. And a microcontroller to tell the motor controllers what to do. Feel free to ask again if you don't know where to go from here!

extreme.aly:

--- Quote from: jwatte on February 17, 2013, 11:32:18 PM ---Right, there are two options:

1) Use four wheels like a model car. Either steer the wheels in front, or drive each wheel individually and do "tank steering." The Wild Thumper does the latter -- check it out here: http://www.pololu.com/catalog/product/1565That, in turn, uses these kinds of motors: http://www.pololu.com/catalog/product/2274

2) Use two big wheels, and two smaller wheels that are just "limp" and ideally pivot. Casters, balls, cart wheels, whatever you want to call them. Turning the big wheels different directions will pivot the robot in place. With only two motors, this is the easiest to implement.

3) You probably want motors with built-in encoders, so you can tell whether one wheel is going faster than the other and compensate, to go straight.

4) You don't need the full weight * radius amount of torque, as long as you don't want to accelerate wildly (and can make soft start/stop part of your control.) Wheels turn very easily, at least if they are narrow and hard.

If you haven't gotten any motors yet, those motors are a fine recommendation for 6V-8V systems. If you are doing a 12V-16V system, try these instead: http://www.pololu.com/catalog/product/1444

You will need some motor controllers to drive them, too. And a microcontroller to tell the motor controllers what to do. Feel free to ask again if you don't know where to go from here!

--- End quote ---

thabk you so much! it really helped me out!-well i cant use more than 2 motors for driving purpose.. its in the rules..-casters.. i heard that the arena that they make is not perfectly levelled (friend's experience), so robots with casters had issues there, like stuck on small bumps.. thats why we dropped that idea..

i'm thinking of buying this motor: http://www.evselectro.com/motors-_vdc-motor-2299well, do i need to have external gears to reduce the speed to my required one? or is PWM good enough?

jwatte:
You need a gearbox. Ideally, a motor with a built-in gearbox, like the ones from Pololu.